Compound transmissions of the range or combined range/splitter type are well known in the prior art. Such transmissions typically comprise a multiple speed main transmission section connected in series with a range type auxiliary section wherein the range step is greater than the total ratio coverage of the main transmission section. Power Take-Off (PTOs) are commonly mounted to the exterior of compound transmissions over an opening in the transmission housing which enables engagement of a transmission gear with a gear in the PTO. PTOs enable directing the power of a truck's engine to auxiliary equipment to perform work at a work site while the truck is stationary. To drive a PTO that is in driving engagement with the range section counter-shaft a main section much have a predetermined main section drive gear engaged. To permit the PTO to be safely operated while the truck is stationary, the range section is placed in a neutral condition.
In compound transmissions, the main section is typically shifted by means of a shift housing assembly controlled by a manually operated shift lever or the like. In contrast, the auxiliary range section is shifted by means of a switch. A switch operated by the operator, either directly or indirectly through the shift linkage and a main section shift mechanism, controls a remote slave valve/actuator mechanism. The valve/actuator mechanism controls operation of a range actuator. The range actuator includes a fluid-actuated piston that divides an actuator cylinder into a low-range and a high-range chamber. The piston moves in response to selective pressurization of one chamber while simultaneously exhausting the other chamber. Since the range section often utilizes synchronized jaw clutches, a range shift is preferably initiated and completed while the main transmission section is in neutral, to provide acceptable shift quality and to prevent undue wear and/or damage to the synchronized jaw clutches.
The neutral condition for the range unit is selected by the vehicle operator when the operator moves a pneumatic switch-valve assembly from a “mobile” position to a “stationary” position.
Prior art compound range type transmissions usually include a control system, which is typically a pneumatic control system having interlock devices, that allows a range shift to be preselected using a selector button or switch at a master control valve. To prevent damage to the jaw clutches, the control system prevents shift initiation of the auxiliary section until the main transmission section is shifted to, or at least towards, the neutral condition. Examples of such transmissions and the control systems therefor may be seen by reference to U.S. Pat. Nos. 2,654,268; 3,138,965, 4,060,005 and 4,974,474. Prior art systems also ensure operator safety by maintaining range unit in a neutral condition while the PTO is operating.
In such transmissions, it is desired to provide a control system which is both highly reliable in maintaining the range section in neutral, at a low component cost.